Bridges are common structures for carrying pedestrian, vehicular, rail traffic and the like over a body of water. If it is neither feasible nor cost effective to construct a bridge, or if an alternate transportation link is desired to alleviate congestion on an existing bridge, an underwater tunnel may be constructed to carry traffic between two land masses separated by a body of water. Underwater tunnels have been constructed by boring a tunnel through the earth beneath the sea bed (“sea” is used herein to refer to any body of water including oceans, lakes and rivers). Underwater tunnels have also been constructed by dredging a trench in the sea bed, lowering preformed tubular sections into the trench, joining the sections together to form one continuous tunnel, backfilling exposed portions of the trench and covering the tunnel with concrete, rock, dirt, mud or other material to hold the tunnel permanently in place on the sea bed. The latter type of tunnel is often referred to as an “immersed tunnel”. Some well-known examples of immersed tunnels include:                the Detroit-Windsor Tunnel spanning the Detroit River and connecting Detroit, Mich., US to Windsor, Ontario, Canada, which opened to traffic in 1930;        the George Massey Tunnel in the Greater Vancouver region of British Columbia, Canada, spanning the south arm of the Fraser River, which opened to traffic in 1959;        the Sydney Harbour Tunnel crossing the Sydney Harbour in Sydney, Australia, which opened to traffic in 1992; and        the Oresund Tunnel, part of the Oresund Tunnel-Bridge connection between Copenhagen, Denmark and Malmö, Sweden, which opened to traffic in 2000.        
Immersed tunnels are often not practical to construct when the bed is too rocky, too deep or too undulating. There is a need for an underwater tunnel which is cost effective to build and can overcome at least some of the disadvantages of existing tunnels. There is also a need for an underwater tunnel which has rotational and lateral stability, and resists tidal currents, earthquakes and tsunamis.